1. Technical Field
This invention relates to downhole working in boreholes, as in the case of oil or geothermal wells.
2. Background Information
During prospecting and production operations, it is often necessary to anchor a tool in a borehole at a chosen depth. More generally, many types of tools are designed to be actuated at a well-determined depth: this is the case, for example, of a cement dump bailer which must be discharged at the depth at which a well is to be closed off.
The conventional procedure consists in first lowering the tool by means of a cable to the desired depth determined by the unreeled length of cable. The tool is then anchored in the production tubing. Then, the actual control of the tool is achieved by repeated pulling exerted from the surface via the cable until the failure of one or more pins. When the cable used is an electric cable, it is possible to use explosive means controlled electrically from the surface. All prior art systems are of the abrupt-action type, which is considered to be necessary in this technique in order to avoid inadvertent triggering of the tool other than at the desired depth.
For traction actuation, the calibration of the fracture pin(s) must be defined carefully and the tool control operations require the securing of the tool in the well. As regards explosive techniques, which are applicable only when an electric cable is used, they also require quite rigorous safety precautions well known to those of the art.
Finally, certain wells having a particular configuration oppose the use of conventional downhole tool triggering techniques. This is the case in particular of wells which exhibit a local restriction beyond which the tool must be triggered. This restriction makes difficult and even impossible the passage of a tool equipped with anchoring means. It may also be mentioned that the control of a tool by pulling on the cable is poorly suited to deviated wells.